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Avalanche Forecast

Archived

Dec 24th, 2011–Dec 25th, 2011

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Northwest Coastal.

Confidence

Fair - Intensity of incoming weather is uncertainfor the entire period

Weather Forecast

Sunday: 5cm of snow - Freezing level at 500m - moderate westerly windsMonday: 20-30cm of snow - Freezing level at 400m - strong southwest windsTuesday: light to moderate snowfall - Freezing level at or around surface - light southwest winds

Avalanche Summary

On the 23rd a natural avalanche cycle took place in the northern part of the region producing slides up to size 3. In the south, observations were extremely limited due to poor visibility. Expect widespread natural avalanche activity throughout the forecast period with expected snow and wind.

Snowpack Summary

Over the past week the forecast region got slammed by snow, strong to extreme wind and fluctuating freezing levels. Including the 18cm that fell on friday night, the southern part of the region has received about 120cm of new snow (Kasiks, Terrace areas). Locations to the north are seeing less (Stewart up to 75cms). Alpine observations have been limited and wind sensors have been disabled by rime, but I suspect new snow has been blown around by strong southwest winds creating wind slabs on lee slopes.In addition to the more obvious direct-action storm instabilities that are expected with the forecast weather, local avalanche professionals have some other concerns: Surface hoar that formed during the winter solstice sits approximately 35cm below the surface and will become more reactive with more wind and snow. As well, the crust-facet combo (extends up to alpine elevations in the south and to 1000m in the north) from the early december dry-spell sits about 150cm below the surface and has not gone away. Any avalanches on this layer would be highly destructive and are probably waiting for the right load or trigger. The mid and lower snowpack are settled out and strong. Snowpack depths in the Kasiks, Terrace areas have reached the 300cm mark. Further north Smithers, Bear Pass depths are ranging from 190-260cms.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.